The present invention relates generally to molding all sorts of materials and more particularly to those synthetic materials known as plastics, whether or not they contain any ancilliary material such as a filler or reinforcing material.
In molding apparatus generally comprising a nozzle and at least one mold to be supplied, the molding material is heated to an elevated temperature in order to plasticize or even liquefy the molding material and then the melt or plasticized molding material is introduced, in practice under pressure, into the mold, taking into account any shrinking during curing, and thereafter the molded material is cooled to reach a sufficiently solid state so that it may retain its molded configuration after unmolding.
In practice, temperature and pressure are employed to give sufficient fluidity to the molding material to enable it to flow through the gate of the mold under satisfactory conditions and then come into intimate mating contact with the cavity of the mold under even more satisfactory conditions. To increase fluidity normally either the pressure or the temperature has to be increased. An increase in the temperature of the molding material is inevitably costly, particularly when the heated molding material has to be subsequently cooled. As for increasing the pressure it is frequently difficult to achieve without considerably complicating the entire apparatus and therefore it is also expensive. Further, whether increasing the temperature or the pressure energy consumption is high.
Under these circumstances it is not unusual, during the cooling of the molding material, notably when it contains fillers or reinforcing materials, for example, glass or carbon fibers, to find in the resulting molded articles, zones of weakness due to a poor distribution of the material and/or premature cooling thereof, sunk spots or voids in the molded material. The zones of weakness, sunk spots or voids, have a tendency to be more common in thin walls, webs and areas of complex configuration which the molded article may include. There is frequently a substantial proportion of rejects of such molded articles.